1. Field of the Invention
The present invention relates to a color wheel suitable as a filter element for a color wheel assembly as a time-share light dispersing device, and a color wheel assembly incorporating such a color wheel.
2. Description of the Related Art
Recently, a projection-type image display apparatus is increasingly coming with a time-sharing single-panel method, in which R (red), G (green), and B (blue) lights are sequentially shed to fall incident on one single light valve element such as a digital micro-mirror device capable of fast switching while the light valve element is driven in synchronization with switching of the incident lights (R, G and B), whereby R, G and B images are produced in a time-series manner and projected sequentially onto a screen, or the like. Here, color composition of the images is accomplished by a viewer due to an afterimage effect occurring at a sense of vision. In the time-sharing single-panel method, reduction in both dimension and weight of the apparatus, which is a feature of a single-panel method, can be achieved by employing a relatively simple optical system, and therefore the time-sharing single-panel method is favorable for realizing inexpensive fabrication of a projection-type image display apparatus. In such an image display apparatus, a color wheel is preferably used as a filter element of a time-share color wheel assembly to sequentially disperse light emitted from a white light source into R, G and B lights having respective wavelength bands in a time-sharing manner.
FIGS. 5A and 5B are respectively top plan and side views of a conventional and typical color wheel assembly 200 as a time-share light dispersing device including such a color wheel. Referring to FIG. 5B, the color wheel assembly 200 comprises a color wheel 100, and a driving motor 106 including a motor hub 105. The color wheel 100 is a tricolor color wheel composed of a disk-like substrate 101 which is made of a light-transmitting material, for example, optical glass, and three pie-shaped (sectorial) filters 102, 103 and 104 which are formed on a surface of the substrate 101, and which transmit exclusively, for example, R, G and B lights, respectively. The color wheel 100 thus structured is fixedly attached to the motor hub 105 coaxially therewith. The color wheel assembly 200 operates such that the color wheel 100 is spun by the driving motor 106 so that white light S impinges sequentially on the filters (R, G and B) 102, 103 and 104 whereby the white light S is sequentially dispersed into R, G and B lights.
There are two types of color wheels. One is a segment-type color wheel comprising a plurality of color filter segments prepared separately and arranged so as to form a disk, and the other is a monolithic-type color wheel structured such that a plurality of color filters are formed on a single disk-like substrate.
FIGS. 6 and 7 are respectively top plan and exploded side sectional views of a segment-type color wheel 160 comprising a plurality (four in FIG. 6) of color filter segments (hereinafter referred to as segments as appropriate) 112, 114, 116 and 118 which are separately prepared and are fixed in position by means of a support member 120 and a clamp member 122 so as to form a disk. The clamp member 122 also functions to fixedly couple the color wheel 160 to the motor hub 105 of the driving motor 106, and a coupling means like the clamp member 122 with such a function is usually used when a monolithic-type color wheel, which has color filters formed integrally on a single substrate thus not requiring a supporting means like the support member 120, is coupled to the motor hub 105 of the driving motor 106.
In such a color wheel assembly, a color wheel must be rotated in synchronization with the operation of a light valve element in order to appropriately produce images as described above, and therefore a detector to detect the rotational position of the color wheel is required. For example, an optical detector for rotational position is provided, in which detection light is radiated onto the color wheel, and variation of reflected light due to the rotation of the color wheel is sensed, whereby the rotational position is detected. In such a detector, a mark member 125, for example a tape, is provided on a portion of the clamp member 122 as shown in FIGS. 6 and 7. The mark member 125 is either larger or smaller in reflectance than the clamp member 122, and a detector (not shown) is disposed so as to direct its detection light to fall incident on the clamp member 122. The detection light is reflected so as to come out with two different intensities alternately when the color wheel 160 is rotated, and the difference in intensity is sensed whereby the rotational position of the color wheel 160 is detected (refer to FIG. 2 in Japanese Patent Application Laid-Open No. H10-48542).
In the conventional color wheel and color wheel assembly as described above, such a mark member is formed by painting, taping, and the like, and additional component and process are required thus incurring a cost increase.